This invention relates to telephone switching systems and more particularly to small electronically controlled switching networks for providing private branch exchange or centralized switching service for electronic key telephone sets.
In the evolution of telephone switching systems, there is now known a type of key system in which only a fixed minimum number of wires need be cabled out to the key telephone set regardless of the number of line pick-up keys with which the set is equipped. This reduction in amount of key set cabling has been made possible by dedicating a pair of wires in the cable to the performance of speech path communication and by recognizing that the control functions, such as transmitting lamp or ringer control signals, operated key button identity and/or call signaling information can be transmitted as data signals over but two other pairs of wires. The operated keybutton identity transmitted over the data link conductor may be employed to control a local switching network to connect the "tip and ring" of the telephone set with the "tip and ring" of the designated line.
The reduction in cabling and the use of data links necessitates that a central or main control unit be provided which, in association with the local switching network, provides the controlling signals for the establishment of the requisite network connections and the transmission to the station sets of the needed control impulses.
The type of switching network which suitably may be employed in providing the centralized switching function for such an electronic telephone system may either be of the space division type as described in L. P. Fabiano U.S. Pat. No. 3,637,939 issued Jan. 25, 1972 or may be of the time division variety. Aspects of an illustrative time division key telephone system are disclosed in D. G. Medill U.S. Pat. No. 3,789,154 issued Jan. 29, 1974; D. G. Medill-J. F. O'Neill U.S. Pat. No. 3,789,152 issued Jan. 29, 1974; and T. G. Lewis U.S. Pat. No. 3,787,631 issued Jan. 22, 1974.
In a time division switching system a frame containing a fixed number of time slots is allocated for use in establishing connections among the station sets or between station sets and central office lines or trunks. The number of time slots determines the number of simultaneous conversations that can take place in the network.
Heretofore there has been some reluctance to employ time division techniques in small switching installations because the systems tend to be rather more expensive to install than space division system. One of the items that has contributed to the expense of time division systems is the need to store in the memory unit associated with the central control information identifying the number of the network time slot assigned to a connection. The storage of a time slot number for a system having a frame of 128 time slot number requires that six bits be provided in the central memory unit for each line, station and trunk having an appearance in the switching network.
Another deterrent to the use of coventional control techniques for time division switching has been that the use of a central time slot assignment memory "map" occasionally introduces a degree of system unreliability. For example, a particular line and station may be assigned a given time slot number so that they may be interconnected by the time division communications bus. The time slot number so assigned usually will, in any well-constructed system, result in that line and station being interconnected during the specified interval. The time slots themselves are defined by a clock and by a counter controlled by the clock. To assign a particular time slot to a line and station in prior art systems the count accruing in the counter during any specified count is entered into a register and the contents of this register are eventually transferred to the central memory where they are stored in the time slot bytes of the line and station activity memory words. The memory elements storing the identities of the line and station and of the time slot thereunto assigned, as well as the register for entering the time slot count accruing in the counter, may typically be constructed of magnetic core elements or of elements employing solid state technology. Both magnetic core and solid state devices are usually quite reliable although each is subject to its particular form of interference. If a stray noise spike is introduced into the system it may, on occasion, inadvertently advance the counter or a memory element may fail to read out properly. If the counter loses synchronism with the clock defining the network time slots actually assigned, a disagreement will result between the actual state of the network and the representation of that state which is reported back to the central control equipment for the switching network. The disagreement between the information stored for the connection in the network map and the connection status reported from the network itself could result in the central control initiating operations to reset the network connections even though the connections themselves are correctly established.
Accordingly, it is an object of the present invention to provide a small time division switching network which dispenses with the need for a centralized memory network map and which is reliable and sufficiently economical for use in small key telephone and private branch exchange installations.